Many modern aircraft, as well as other vehicles and industrial processes, employ gas turbine engines for generating energy or propulsion. Such engines generally include a fan, a compressor, a combustor and a turbine arranged in that order from first to last along a central longitudinal axis.
In operation, atmospheric air enters the gas turbine engine through the fan and at least a portion of that air passes through the compressor and is pressurized. The pressurized air is then mixed with fuel in the combustor. Within the combustor, the fuel-air mixture is ignited, generating hot combustion gases that flow axially to the last stage of the core, i.e., the turbine. The turbine is driven by the exhaust gases and the turbine's rotation mechanically powers the compressor and fan via a central rotating shaft, maintaining the combustion cycle. After passing through the turbine, the exhaust gas exits the engine through an exhaust nozzle.
While a portion of the incoming atmospheric air passes through the compressor, combustor and turbine as discussed above, another portion of the incoming air may pass only through the fan before being routed around the core. This air “bypasses” the core, but provides thrust nonetheless due to being accelerated by the fan and routed through the engine nacelle (outside the core). The engine may be optimized to provide either thrust (e.g., with a substantial bypass around the core) or shaft power (e.g., with no bypass and an efficient power-absorbing turbine) depending upon the intended application of the engine.
In either arrangement, the high compression stage of the engine feeds into the combustor within the core. At the junction between the high compression stage and the combustor, a static exit guide vane (EGV) assembly minimizes rotation and turbulence that was introduced into the airflow by the compressor stage. From the EGV assembly, a pre-diffuser expands and slows the airflow entering the combustor.
However, as the gas turbine engine operates, the various engine components absorb different amounts of heat energy, which may in turn cause different rates of thermal expansion in the components. This problem is particularly acute between the EGV assembly, which has low mass and thin elements, and the pre-diffuser, which is significantly more massive. The differential in thermal expansion rates can lead to disproportionate stresses on the less massive EGV assembly, which may then require frequent checking, repair and replacement.